The present invention relates to an objective, an optical pickup apparatus and an optical information recording and/or reproducing apparatus.
In recent years, high density optical information recording media (which are also called optical discs) whose recording density has been improved by using a blue laser light source has been put to practical use. For the purpose of realizing simplification of the structure of an optical pickup apparatus, low cost and compactness, an objective having compatibility for a high density optical disc, DVD and CD is needed.
An objective that is used for an optical pickup apparatus having compatibility for the three types of optical information recording media stated above is disclosed in the following Document 1.
Document 1: TOKUKAI No. 2004-265573 (Japanese Published Patent Application No. 2004-265573)
The objective disclosed in numerical Example 2 of the aforesaid Document 1 has a diffractive structure that generates second order diffracted light for a blue laser light flux and generates first order diffracted light for an infrared laser light flux for CD and for a red laser light flux for DVD, and diffractive actions of the diffractive structure correct spherical aberration caused by a protective substrate thickness difference between a high density optical information recording medium and DVD, and correct spherical aberration caused by a protective substrate thickness difference between a high density optical information recording medium and CD by making a divergent light flux enter the objective for conducting recording and/or reproducing of information for CD.
However, this objective has the following two problems. One of them is one that the wavelength-dependency of the spherical aberration caused by the diffractive structure is high. In this case, a laser light source wherein an emission wavelength is deviated from the design wavelength cannot be used, and a laser light source needs to be selected, which increases manufacturing cost of the optical pickup apparatus. Since the spherical aberration grows greater in proportion to the fourth power of the numerical aperture, an influence of the wavelength-dependency of the spherical aberration for the diffractive structure is great, when the Blue-ray disc (BD) that uses an objective whose numerical aperture is 0.85. Another problem is that satisfactory recording and/or reproducing characteristics cannot be obtained for CD, because a degree of divergence of the infrared laser light source is too strong, and thereby, occurrence of coma in the case of tracking by the objective is too great, in the case of conducting recording and/or reproducing of information for CD.
An angle of diffraction of the diffracted light is expressed by “diffraction order×wavelength/diffraction pitch”. For realizing compatibility between optical information recording media wherein a working wavelength is different each other, by utilizing diffractive actions, a prescribed difference needs to be given to the angle of diffraction. Both of the two problems mentioned above are caused by using the diffractive structures wherein values of “diffraction order×wavelength” are substantially the same between respective working wavelength for each optical information recording media.
In numerical Example 2 of the aforesaid Document 1, since “diffraction order×wavelength” ratio of blue laser light flux to red laser light flux is 810/650=1.25 which is close to 1 (wherein a unit of the wavelength is nm), the diffraction pitch needs to be small for obtaining a difference of necessary angle of diffraction for correcting spherical aberration caused by a protective substrate thickness difference between a high density optical information recording medium and DVD. Accordingly, the wavelength-dependency of spherical aberration of the diffractive structure grows greater, and “problem to select laser light source” becomes obvious. Further, since the degree of difficulty of processing a metal mold for the diffractive structure is enhanced, it becomes difficult to form an accurate diffractive structure.
On the other hand, since “diffraction order×wavelength” ratio of blue laser light flux to infrared laser light flux is 810/780=1.03, and an angle of diffraction of the blue laser light flux is substantially the same as that of infrared laser light flux, it is impossible to correct spherical aberration caused by a protective substrate thickness difference between a high density optical disc and CD, by using diffractive actions. Accordingly, for correcting spherical aberration caused by a protective substrate thickness difference between high density optical information recording medium and CD, magnification in the case of using high density optical information recording medium is required to be different from that in the case of using CD. As a result, “problem of tracking characteristics” becomes obvious.
In view of the problems mentioned above, the invention has been attained, and its objective is to provide an objective which can conduct properly spherical aberration correction necessary for attaining compatibility among high density optical disc, DVD and CD, without making a sacrifice of various characteristics such as wavelength-dependency of spherical aberration and tracking characteristics, an optical pickup apparatus employing the aforesaid objective and an optical information recording and/or reproducing apparatus in which the optical pickup apparatus is incorporated.